HAMBURG, Germany — In a surprising U-turn, German officials said initial tests published Monday provided no evidence that sprouts from an organic farm in northern Germany were the cause of the country's deadly E. coli outbreak.

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The Lower-Saxony state agriculture ministry said 23 of 40 samples from the sprout farm suspected of being behind the outbreak have tested negative for the highly aggressive, "super-toxic" strain of E. coli bacteria. It said tests were still under way on the other 17 sprout samples.

"The search for the outbreak's cause is very difficult as several weeks have passed since its suspected start," the ministry said in a statement, cautioning that further testing of the sprouts and their seeds was necessary to achieve full certainty.

The waffling over possible causes of the outbreak points to a flawed investigation that could shake faith in the global public health system, a top U.S. food safety official said Monday.

"All this wishy-washy back-and-forth, it's just incompetence," said Dr. Michael Osterholm, director of the Center for
Infectious Disease
Research and Policy at the University of Minnesota. "Where's the epidemiology?"

The ministry statement about samples from the Gaertnerhof organic sprouts farm in the northern German village of Bienenbuettel left consumers across the continent still puzzled as to what is safe to eat. The ministry itself also said it was not clear how soon an answer would be found.

Poly tunnels at a vegetable farm is seen in the Uelzen district, northern Germany. The farm was shut down Sunday and all of its produce -- including fresh herbs, fruits, flowers and potatoes -- was recalled. At least one of the farm's employees was also infected with the E.coli bacteria, the minister said.

"A conclusion of the investigations and a clarification of the contamination's origin is not expected in the short term," the ministry added.

According to NBC's Michelle Kosinski, investigators are returning to the suspected sprout farm in Bienenbuettel Monday afternoon to do more tests.

The current crisis is the deadliest known E. coli outbreak, killing at least 22 people and sickening more than 2,300 across Europe.

Suspicion for the cause of the E. coli outbreak had initially fallen on contaminated cucumbers from Spain, but researchers then concluded that the cucumbers were contaminated with a different strain of E. coli.

German authorities on Sunday issued a warning against eating any sprouts and kept up their earlier warning against eating tomatoes, cucumbers and lettuce.

In Germany alone, 2,231 people have been infected since May 2, with 630 of them suffering from a rare, serious complication that can lead to kidney failure, Germany's national disease control center said Monday.

A Michigan resident who recently returned from northern Germany is among four people in the U.S. apparently sickened by the severe E. coli strain, a state health official said.

The U.S. Centers for Disease Control and Prevention announced Friday that three of the four had been hospitalized with kidney failure, a hallmark of the E. coli outbreak that has sickened more than 2,200 people.

The person from Michigan is recovering in the southeastern portion of the state, Department of Community Health spokeswoman Kelly Niebel told The Detroit News for a story published Monday.

Preliminary epidemiological tests had found that sprouts from the Gaertnerhof Bienenbuettel farm could be traced to infections in five German states. Many restaurants had received deliveries of the sprouts, which are often used in mixed salads.

Sprouts have also been implicated in previous E. coli outbreaks, particularly one in 1996 in Japan, in which tainted radish sprouts killed 12 people and reportedly sickened more than 9,000.

E. coli is found in the feces of humans and livestock and can spread to produce through sloppy bathroom habits among farmworkers or animal waste in fields and in irrigation water.

Mounting losses for farmers
The manager of the suspected farm in Bienenbuettel said he could not understand how it could be the source of an infection that is usually transmitted through feces, or food or water contaminated with fecal bacteria.

The Shiga toxin-producing E.coli (STEC) strain found in this outbreak is known to be able to lurk in cows' intestines.

"I can't understand how the processes we have here and the accusations could possibly fit together," Klaus Verbeck told the regional newspaper Neue Osnabruecker Zeitung.

"The salad sprouts are grown only from seeds and water, and they aren't fertilized at all. There aren't any animal fertilizers used in other areas on the farm either."

The relief in Germany that investigators had found a possible source of the killer bacteria -- ironically in bean sprouts, eaten by many as "health food" -- was tempered by the cautious tone of the ministry statement, and by reports of mounting losses for vegetable farmers and retailers across Europe caused by three weeks of panic.

In Brussels, the European Commission said it would hold a special meeting of EU farm ministers in Luxembourg Tuesday. One EU source told Reuters the ministers would discuss financial aid to fruit and vegetable producers hit by the E.coli crisis.

Scientists say the contamination may have been on or in the bean seeds themselves, in the water used to grow them, or have come from a worker handling them

Officials at the U.S. Centers for Disease Control and Prevention say two previous reports of a similar strain have occurred elsewhere. One involved a 29-year-old woman in South Korea, reported in 2006. The other was a small cluster of cases in the Republic of Georgia in 2009.

The Associated Press, Reuters, NBC and msnbc.com's JoNel Aleccia contributed to this report